Average Speed and Force Question

[baltar]

Hello everyone! I am really terrible at physics.
I genuinely tried doing them(looking for similar examples in the book and such), but am still stuck. If someone could point me in the right direction, I'd appreciate it. Thanks in advance!

1) A car is making a 12 mile trip. It travels the first 8.0 miles at 30mph and the last 4.0 miles at 60mph. What is the car's average speed for the entire trip?
I got 40mph. This is what I did 0.66666(30) + 0.33333(60) = 40mph.


2) A car is moving with constant acceleration. At t = 5.0 its velocity is 8.0m/s and t=8.0s its velocity is 12.0m/s. What is the distance traveled in that interval of time?

3) To move a large crate across a rough floor, you push on it with a force F at an angle of 21 degrees below the horizontal. Find the force necessary to start the crate moving, given that the mass of the crate is 32 kg and the coefficient of static friction between the crate and the floor is 0.57

 

[Integral]

1. Use your knowledge of speeds and distance to compute the total time for the trip. Use that time and the total distance to get a average velocity.

2. and 3. Show us something of what you know about these situations.

 

[baba89]

Hello there! It's great that you're trying to understand these physics concepts. Don't worry, with some practice and guidance, you'll get the hang of it. Here are some tips to help you with these problems:

1) For the first question, remember that average speed is calculated by dividing the total distance traveled by the total time taken. In this case, the total distance is 12 miles and the total time is 0.66666 hours (since the car traveled 8 miles at 30mph, which takes 0.66666 hours and 4 miles at 60mph, which takes 0.33333 hours). Therefore, the average speed would be 12 miles divided by 0.66666 hours, which gives us 18 miles per hour (not 40mph).

2) For the second question, you can use the formula v = u + at, where v is the final velocity, u is the initial velocity, a is the acceleration, and t is the time. Here, you have the final and initial velocities, and you need to find the distance traveled. Rearrange the formula to solve for distance (s) and substitute the given values to find the distance.

3) For the third question, you can use the formula F = μN, where F is the force required to start the crate moving, μ is the coefficient of static friction, and N is the normal force (which is equal to the weight of the crate). You can find the weight of the crate by multiplying its mass by the acceleration due to gravity (9.8 m/s^2). Then, use trigonometry to find the normal force, and finally, substitute the values into the formula to find the force required.

I hope this helps! Keep practicing and don't be afraid to ask for help if you're still stuck. Good luck!